1. Field of the Invention
The present invention relates to an exhaust gas purification system and an exhaust gas purification method for effectively purifying harmful substances present an exhaust gas, particularly hydrocarbons (HC) and the like produced in large quantities at the time of a cold start of an engine.
2. Description of the Related Art
According to an FTP test (LA-4 mode) in correspondence with the intensification of exhaust gas regulations in the United States, 70 to 80% of the total HC emission amount is emitted within 140 seconds (within the first hill of Bag 1) at the time of a cold start of an engine after engine cranking. During this period, however, considerable time is taken until a conventional three-way catalyst has reached its operation temperature, and thus most of the HC is exhausted without being purified.
In order to solve this problem, a technique has been proposed in which an electrically heated catalyst (hereinafter referred to as an EHC) and an adsorbent mainly comprising zeolite are disposed in an exhaust gas pipe.
For example, in Japanese Patent Application Laid-open No. 31359/1993, a catalyst converter for car exhaust gas purification has been disclosed in which three elements including an EHC, an adsorbent and a main catalyst are arranged in series in an optional order in an exhaust gas flow path.
Additionally, SAE paper No. 920847 has suggested a by-pass system in which a valve is provided on the upstream side of an adsorbent so as to desorb HC after a catalyst has been sufficiently activated.
However, Japanese Patent Application Laid-open No. 1359/1993 does not disclose any optimum arrangement of the above-mentioned three factors for the sake of the decrease of an emission at the time of a cold start. Further, with regard to the minimization of electric power to be applied to the EHC, no consideration has been given.
Moreover, the technique employing such a by-pass system as in the SAE Paper No. 920847 suitably functions from a viewpoint of purification performance, but it has many problems from a practical viewpoint. For example, this system is intricate, and the reliability of the heat-resistant valve in the pipe is poor.